1. Field of the Invention
The present invention relates to a photosensitive member for electrophotography, which is used in an image-forming device for forming images by electrophotography and, in particular, to a photosensitive member in a laser printer or the like device to be driven with a light source of a semiconductor laser. 2. Description of the Related Art
Recently, a photosensitive member having a germanium-containing amorphous silicon (hereinafter referred to as "a-SiGe") layer as a photoconductive layer or that having a laminate layer composed of amorphous silicon (hereinafter referred to as "a-Si") and a-SiGe as a photoconductive layer is being desired to be put into practical use as a photosensitive member in a laser printer or the like device to be driven with a light source of a semiconductor laser, because of the following advantageous merits:
(1) The member has a long life.
(2) The member is non-toxic to human bodies.
(3) The member is highly sensitive to lights having a long wavelength.
In the photosensitive member having the above-mentioned constitution, both the a-SiGe layer and the a-Si layer are formed by a so-called plasma CVD method (p-CVD) or sputtering method in the prior art. Therefore the total content of hydrogen and halogen in the a-SiGe and a-Si layer is limited at most and is generally within the range of from 10 to 40 at.% (U.S. Pat. Nos. 4,265,991 and 4,490,450).
The maximum oscillating wavelength capable of stably giving a high output power with a semiconductor laser which is now practically used is from 780 to 830 nm. However, a Ge atom-free a-Si photosensitive member could not have a sufficient sensitivity to the light having a wavelength falling within the range. Therefore, if the member is to be used as a photosensitive member in a laser printer or the like device to be driven with a light source of a semiconductor laser, some pertinent changes must be imparted to the member.
On the other hand, as containing Ge, the a-SiGe layer may have a reduced optical band gap and can therefore have an improved sensitivity to lights having a long wavelength. However, since the total content of hydrogen and halogen in the layer is strictly limited to fall within the range of from 10 to 40 at.%, the layer is to have a small dark resistivity and have an extremely lowered charge-retention capacity. Additionally, the layer cannot have a sufficient light-sensitivity as a whole.
In order to overcome the problem of the lowered charging characteristic, which is one drawback of the photosensitive member having a photoconductive layer composed of only the a-SiGe layer, one means has been proposed in which an a-Si layer to have a function essentially as a charge-retention layer and a charge-transportation layer is employed and the a-Si layer is laminated below the a-SiGe layer (in the side near to the support) to thereby reduce the thickness of the a-SiGe layer to inhibit the decrease of the dark resistivity.
However, even in the photoconductive layer having such a laminate structure (composed of a-SiGe layer in the surface side of the photosensitive member and a-Si layer in the side near to the support), improvement of the charging characteristic is still insufficient. The reason would be as follows:
In the a-Si layer and a-SiGe layer, the carrier is essentially thermally excited rather than the band-to-band level, and after the excited carrier the band-to-band level is to have a charge with a polarity opposite to the excited carrier. In the case of positive charge, since the a-Si layer and a-SiGe layer to be employed as a photoconductive layer are made to be p-type ones in consideration of the running capacity of the carrier, a positive carrier with a polarity which is same as the positive charge is excited and the band-to-band level is to have the remaining negative charge. The phenomenon is same as that in which the negative charge as excited in the support to the positive surface charge is introduced into the position of the excited positive carrier. Therefore, where an a-SiGe layer which has a large amount of heat-excited carriers is provided near to the surface layer of a photosensitive element, the charge characteristic of the member would be noticeably deteriorated.
Moreover, as already mentioned above, both the a-Si layer and the a-SiGe layer have a total amount of hydrogen and halogen of being 40 at.% or less. Therefore, the photosensitive member is to have a small resistivity and have a poor charging characteristic.
In order to overcome the problem, it may be considered to reverse the order for lamination of the layers, or that is, the a-SiGe layer is provided to the side of the support and the a-Si layer to the surface side. However, in order to at least maintain the function as a photoconductive layer, the layer over the a-SiGe layer is necessary to have a good light-transmittance to the incident rays having a wavelength of from 680 to 730 nm. Despite of the necessity, the conventional a-Si layer has a total amount of hydrogen and halogen of being 40 at.% or less and therefore has a small optical band gap of being approximately 1.7 eV and, as a result, it does not satisfy the light-transmittance requirement. Accordingly, the indicent rays would be absorbed by the band-to-band level of the a-Si layer and therefore could not act as a carrier and to thereby cause lowering of the light-sensitivity of the photosensitive member as a whole.
On the other hand, in accordance with a conventional method of forming onto conductive layer, the deposition rate for the layer and the gas-utilizing efficiency are both low. In particular, where an expensive gas such as GeH.sub.4 or Ge.sub.2 H.sub.6 is used as a raw material gas in forming a-SiGe layer, the cost of the photosensitive member to be formed is extremely high. Moreover, if the deposition rate is to be increased in the conventional method, generation of a polymer powder which consists essentially of (SiH.sub.2).sub.n is inevitable. The polymer powder would adhere to the substrate of the photosensitive member during the deposition thereon to interfere with a normal growth of the film being formed. As a result, the photosensitive member obtained is to be a non-conforming product. From the point, the cost of the photosensitive member product to be formed by the conventional method is to be extremely high.
In conclusion, a photosensitive member having an a-SiGe layer as formed by the conventional plasma CVD method or sputtering method which has a limited content of hydrogen and halogen of being from 10 to 40 at.% or having a laminate layer composed of the a-SiGe layer and an a-Si layer also with a limited content of hydrogen and halogen of being from 10 to 40 at.%, as a photoconductive layer is far from practical use in view of the electrical characteristics and the cost thereof, irrespective of the order of lamination of the two layers.
It is known to use an a-Si layer or a-SiN layer as formed by electron cyclotron resonance method as a photoconductive layer of a photosensitive member for electrophotography (U.S. Pat. No. 4,532,199), but it is unknown to use an a-SiGe layer formed by the same method.